Method of producing vacuum in the production of polymers

ABSTRACT

The invention relates to a method for the production of polymers by means of melt-phase polycondensation in at least one reaction step under vacuum. The vapor produced is drawn off by at least one steam jet vacuum pump with a downstream injection condenser from the reaction step. In order to improve the method, the steam jet vacuum pump is driven with alkylene carbonate in vapor form and liquid alkylene carbonate is supplied to the injection condenser as coolant.

The invention relates to a method for producing vacuum and forseparating condensable components in the production of polymers such aspolyester, polyarylate, polyphosphonate, polysulfone, polyetherketoneand polycarbonate by means of melt-phase polycondensation ofaccumulating vapour in at least one reaction step under vacuum, whereinthe suction side of the reaction step is connected to at least one steamjet vacuum pump with a downstream injection condenser.

As a result of their exceptional mechanical-technological properties,polymers are widely used in the mechanical engineering industry, inelectrical engineering, in the building trade, in the textile industry,in the paints industry and for objects of everyday use. Production takesplace either by interface condensation or melt polycondensation bydirect polycondensation from dicarboxylic acids and dialcohols ordiphenols or by transesterification of the corresponding acid esters.When melt polycondensation is used to produce polycarbonates andpolyphosphonates, aromatic dihydroxy compounds, e.g.bis(4-hydroxyphenyl)alkanes, especially bisphenol A, are transesterifiedwith diphenyl carbonate or diarylalkyl phosphonates in the presence ofcatalysts with the separation of phenol, oligomerised and then subjectedto polycondensation. The polycondensation takes place in severalreaction steps under increasing vacuum, for example, beginning with agentle vacuum of |800| mbar, a vacuum of <|100| bar is set for thepre-polycondensation, and a vacuum of <|1| mbar is set in the end stepfor the polycondensation at a temperature of 220 to 350° C.

When the vacua are produced by means of positive-displacement vacuumpumps followed or preceded by surface condensers to separate thecondensable components contained in the vapours produced during thepolycondensation, such as phenols, multivalent alcohols, monomer oroligomers, the condensable components are deposited at suitably lowcondensation temperatures in the surface condensers and/or in the pumpand piping system so that interruptions of operation occur. To avoidthis disadvantage, coolers with rotating scrapers have been proposed toclean the cooling surfaces. A disadvantage however is that in the eventof a leak from the shaft passage which is under vacuum, there is a majorrisk for operation and product quality. It is also known (SRI Report No.50B [1982] Polycarbonates, FIG. 5.1) to produce the vacuum by means oftwo water-vapour operated steam-jet vacuum pumps arranged one after theother in the final stage of the polycondensation whilst retaining thesurface condenser. In this case, the waste water is contaminated, forexample, by the accumulating phenols, dialcohols and oligomers. Inaddition, the oligomers are deposited in the steam-jet vacuum pumps.

U.S. Pat. No. 3,468,849 and DE-A-2227261 describe methods for producingpolyethylene terephthalate (PET) wherein the vacuum is produced in thefinal stage of the polycondensation by means of a steam-jet vacuum pumpdriven by ethylene glycol (EG) vapour at a pressure of approx. |2| bar.EG is liquid at room temperature and boils at a pressure of |2| bar anda temperature of 222° C. whereas the monomeric initial products of theproduction of polycarbonates, polysulfones, polyether ketones,polyarylates and polyphosphonates are solid at room temperature and havehigh boiling points of >300° C. at atmospheric pressure; in this case,partly undesirable decomposition and side reactions occur. In thepolycondensation of PET, the EG which is both the working medium of thesteam-jet vacuum pump and the monomer is separated and accumulates undernormal conditions in liquid form.

During the polycondensation of polycarbonates from diphenyl carbonate,of polyarylates from dicarboxylic acids, of polyphosphonates frombisphenols and/or other phenyl esters, various phenols are formed fromthe monomers or the separation products from the transesterificationreaction in the production of polyesters, which are toxic and becomesolid at a temperature below 41° C. These properties impair the safetyof the operation and staff and cause corrosion effects.

The subject matter of DE-A-4440741 is a method for producing a vacuumand separating the condensable components from the vapours of melt-phasepolycondensation in the production of polycarbonate. In this method thesuction side of a polycondensation stage is connected to at least one totwo steam-jet vacuum pumps with a downstream injection condenser. Theworking vapour is diphenyl carbonate vapour at a pressure of |0.3| to|1.5| bar and the spray liquid is liquid diphenyl carbonate. This methodmakes it possible to advantageously produce vacuum in one or morereaction stages during the production of polycarbonate by melt-phasepolycondensation. Operating interruptions caused by deposition ofoligomers or by saturation of surface condensers are almost eliminated.Emissions to be disposed of are reduced to a minimum, in particular nowaste water contaminated with phenol, oligomers and monomersaccumulates.

Starting from this prior art, it is the object of the present inventionto improve the energy efficiency of the method described initially.

This object is solved by the fact that the steam-jet vacuum pump isacted upon by working vapour consisting of at least one alkylenecarbonate in vapour form and/or at least one dialkyl carbonate in vapourform and the injection condenser is acted upon by coolant consisting ofat least one liquid alkylene carbonate and/or at least one liquiddialkyl carbonate.

Despite a melting point of about 37° C. and a boiling point of about248° C. at a pressure of |1| bar, ethylene carbonate is quite especiallysuitable for solving the object according to the invention in the vapourform as working vapour for the steam-jet vacuum pump and in the liquidform as spray agent for the injection condenser for spray condensationof the high-boiling components of the vapours. It was surprisingly foundthat ethylene carbonate is suitable for undercooling and thus can beused in vacuum production installations similarly to the alkylcarbonates such as propylene and butylene carbonate.

The working vapour used for the operation of the steam-jet vacuum pumphas a pressure of |0.3| mbar to |9| bar. In order to achieve arelatively high efficiency, the working vapour should have the highestpossible pressure. Depending on the dimensions of the installation forproducing polyester, polyarylate, polyphosphonate, polysulfone,polyether catone or polycarbonate, a thermally sparing mode of operationmay be appropriate at a pressure of |0.5| mbar to |1| bar. At relativelyhigh pressures the use of one steam-jet vacuum pump per reaction stageis generally sufficient whereas at relatively low pressures twosteam-jet vacuum pumps are inserted before an injection condenser. Thetemperature of the working vapour corresponds to the boiling point ofthe ethylene, propylene and alkylene carbonate at given pressure.

In order to avoid separation of condensable components of the vapours inthe steam-jet vacuum pump, it is appropriate to overheat the workingvapour before entry into the steam-jet vacuum pump by 1 to 100° C.,preferably 3 to 25° C.

The vapour mixture leaving the steam-jet vacuum pump, consisting ofworking vapour, the separation products of the polycondensation andtransesterification such as phenol, dialcohol and other substances andoligomeric polycarbonate contained in the polycondensation vapours,other oligomeric polymer and monomers, is fed into the injectioncondenser directly adjacent to the steam-jet vacuum pump, and thecondensable components are separated therefrom by spraying with liquidalkylene carbonate. Depending on the purity of the spraying agent, itstemperature should be as low as possible, at 10 to 200° C., preferably25 to 150° C., in order to achieve maximum separation of the condensablecomponents.

Within the scope of the embodiment of the invention, a partial mass flowof the condensate flowing away from the injection condenser is fed backinto the injection condenser as spraying agent under suitable tempering,a further partial mass flow is fed to the evaporator to produce theworking vapour and the residual mass flow is discharged and fed backinside the process or supplied to a recovery plant. An enrichment ofoligomers, monomers and separation products of the polycondensation ortransesterification is thereby avoided. The condensates of a pluralityof injection condensers can initially be brought together and only thendivided into the corresponding partial mass flows.

The vapour phase emerging from the first injection condenser has ahigher pressure corresponding to the compression ratio of the previouslyincorporated steam-jet vacuum pump(s) compared with the pressure in thepolycondensation stage. Further compression of the vapour phase can beprovided by one or a plurality of subsequent compression stages eachformed of a steam-jet vacuum pump and injection condenser. It is alsopossible to replace one or a plurality of the subsequent compressionstages consisting of a steam-jet vacuum pump and injection condenser bymechanical vacuum pumps such as a vacuum blower and liquid ring pump.Condensate from the injection condenser or a liquid alkylene carbonateor a mixture of liquid alkylene carbonates or dialkyl carbonates can beused as operating liquid for the liquid ring pump. If necessary, heatexchangers and/or condensers can be interposed. At the same time, thevacuum for the preceding polycondensation stages can be produced usingthese further compression stages. If mechanical vacuum pumps are used,these can also be used to simultaneously produce the vacuum of thetransesterification stage. A further partial flow of condensate from theinjection condensers is supplied to the evaporator to produce theworking vapour for the steam-jet vacuum pumps. The evaporator can alsobe operated exclusively using pure alkylene carbonate without returningcondensate. The operating pressure of the evaporator is slightly higherthan the pressure of the working vapour depending on the pressure lossin the pipes and fittings. In the case of a plurality of compressionstages, the working vapour flow emerging from the evaporator is dividedinto partial flows corresponding to the number of compressor stages. Thesump of the evaporator is continuously partially discharged andoptionally reused.

Since the alkylene carbonates used in this method have a substantiallyhigher boiling point than the phenols used, simple distillativeseparation is possible. The method according to the invention allowsundisturbed and economical production of vacuum for one or a pluralityof polycondensation reaction stages in the production of polyester,polyphosphonate, polysulfone, polyether ketone, polyarylate andpolycarbonate by melt-phase polycondensation. Operating interruptionscaused by deposition of oligomers are almost eliminated. All valuablesubstances are recirculated within the process. Emissions to be removedare reduced to a minimum, and in particular no waste water contaminatedwith phenol, oligomers and monomers accumulates.

The method according to the invention is explained in detail and as anexample by the flow pattern shown in the drawing.

The product removed from a pre-polycondensation reactor not shown issupplied via pipe (1) to the polycondensation reactor (2) where atemperature of 295° C. and a pressure of |2| mbar prevails; the polymeris led out from the polycondensation reactor (2) via pipe (3). Thedelivery side of the polycondensation reactor (2) is connected via line(4) to the steam-jet vacuum pump (6) provided with heat tracing (5),which is driven by ethylene carbonate vapour supplied via line (7) fromthe evaporator (8). In order to avoid deposition of components presentin the ethylene carbonate vapour in the area of the steam-jet vacuumpump (6), the ethylene carbonate vapour can be overheated using heattracing (9) surrounding the line (7). The vapour mixture of ethylenecarbonate vapour leaving the steam-jet vacuum pump (6) and thepolycondensation vapours containing the phenol, oligomeric polycarbonateand monomers is fed into a directly adjacent injection condenser (10)where the condensable components are separated by spraying with liquidethylene carbonate having a temperature of 40° C. supplied via the line(11). The condensate removed from the injection condenser (10) via line(12) flows into a condensate collector (14) provided with heat tracing(13). The vapour phase flowing out at the head of the injectioncondenser (10) flows via line (15) to the steam-jet vacuum pump (17)provided with heat tracing (16), which is driven by ethylene carbonatevapour supplied via the line (18) which branches off from the line (9).The vapour mixture of ethylene carbonate vapour leaving the steam-jetvacuum pump (17) and the phenol-containing polycondensation vapour issupplied to an injection condenser (19) and sprayed therein withcondensate supplied via line (20). The condensate leaving the injectioncondenser (19) via line (21) flows into the condensate collector (14).The condensate flowing from the condensate collector (14) via line (22)is fed to the injection condensers (10) or (19) via lines (11) or (20)using the pump unit (23). The vapour phase emerging from the head of theinjection condenser (19) flows via line (24) to a liquid ring pump (25)by which means the waste gas is removed from the process via line (26).

1. A method for producing vacuum and for separating condensablecomponents in the production of polymers such as polyester, polyarylate,polyphosphonate, polysulfone, polyetherketone and polycarbonate by meansof melt-phase polycondensation of accumulating vapor in at least onereaction step (2) under vacuum, wherein the suction side of the reactionstep is connected to at least one steam jet vacuum pump (6, 17) with adownstream injection condenser (10), characterized in that the steam-jetvacuum pump (6, 17) is acted upon by working vapor consisting of atleast one alkylene carbonate in vapor form and/or at least one dialkylcarbonate in vapor form and the injection condenser (10, 19) is actedupon by coolant consisting of at least one liquid alkylene carbonateand/or at least one liquid dialkyl carbonate.
 2. The method according toclaim 1, characterized in that in a melt-phase polycondensation whichtakes place in a plurality of reaction steps (2) the last reaction stepis connected to a steam-jet vacuum pump (6, 17) with a downstreaminjection condenser (10, 19).
 3. The method according to claim 1,characterized in that in a melt-phase polycondensation which takes placein a plurality of reaction steps (2), the last reaction step and atleast one of the preceding reaction steps is connected to a steam-jetvacuum pump (6, 17) with a downstream injection condenser (10, 19). 4.The method according to claim 1, characterized in that preferablyethylene carbonate and propylene carbonate is used as alkylenecarbonate.
 5. The method according to claim 1, characterized in thatpreferably dimethyl carbonate and diethyl carbonate are used as dialkylcarbonate.
 6. The method according to claim 1, characterized in that thepressure of the working vapor is |0.3| mbar to |9| bar, preferably |5|mbar to |1| bar.
 7. The method according to claim 1, characterized inthat the vapor-phase working medium is overheated before entering intothe steam-jet vacuum pump (6, 17) by 1 to 100° C., preferably 3 to 25°C.
 8. The method according to claim 1, characterized in that thetemperature of the coolant is 10 to 200° C., preferably 25 to 150° C. 9.The method according to claim 1, characterized in that the delivery sideof at least one injection condenser (19) is connected to apositive-displacement vacuum pump (25), preferably a liquid ring pump.10. The method according to claim 1, characterized in that from theaccumulating quantity of condensate, a first partial quantity is fedback into the injection condenser (10, 19), a second partial quantity isfed to an evaporator (8) to produce the working vapor and the residualquantity is returned inside the process or fed to a recovery plant.